1. Field of the Invention
This invention relates to a method for removing phosphorus from water to be treated. More concretely, the invention relates to a method for removing phosphorus from water to be treated, wherein an electric field is formed in the water to be treated to accumulate hydroxide ions or an electrolysis is conducted to generate hydroxide ions, whereby phosphate ions in the water to be treated are precipitated as a salt slightly soluble in water.
2. Description of Related Art
As the conventional method for removing phosphorus from water to be treated, there are known a method of adding a flocculant such as calcium hydroxide, an iron salt, an aluminum salt or the like, a method of adding an adsorbent such as a clay mineral, alumina or the like, a method of conducting absorption with a multiplying microbe, and so on. In these methods, however, operations become complicated and an amount of sludge generated is large, so that the application to a real-scale water treatment equipment is few. On the other hand, there is a method wherein a few amount of a seed crystal of calcium is added to precipitate a hydroxy apatite as the method for the removal of phosphorus decreasing the generation of the sludge, but this method is not yet put into practical use due to problems in the treating stability to an actual waste water and the like.
It is, therefore, an object of the invention to solve the above problems and to provide a novel method for removing phosphorus from water to be treated, wherein the treating operation is very simple and the removal of phosphorus can be conducted in a higher efficiency without adding chemicals.
The inventors have made various studies for solving the above-mentioned problems and as a result, the invention has been accomplished. According to the invention, there is the provision of a method for removing phosphorus from water to be treated, which comprises immersing electrodes into a tank filled with the water to be treated, applying a DC voltage between the electrodes to generate hydroxide ions through a cation migration in the water and/or an electrolysis, and precipitating phosphate ions in the water as a salt slightly soluble in water.
There are the followings as a preferable embodiment of the invention:
(1) The tank is filled with granular solids;
(2) The electrodes are a multi-electrode system;
(3) The cation migration and accumulation and/or the electrolysis are conducted while flowing the water to be treated through the tank;
(4) The granular solids are sands, glass beads, or shells;
(5) The multi-electrode system is porous or meshed;
(6) The water to be treated contains calcium ions and/or magnesium ions; and
(7) A phosphorus compound removed from the water to be treated is recovered by back wash.
Moreover, optional combinations of the above items (1)-(7) are also preferable, otherwise specified.